[unittest] Explicitly specify alignment when using BumpPtrAllocator.

r297310 began inserting red zones around allocations under ASan, which
perturbs the alignment of subsequent allocations. Deliberately specify
this in two places where it matters.

Fixes failures when these tests are run under ASan and UBSan together.
Reviewed by Duncan Exon Smith.

rdar://problem/30980047

llvm-svn: 297540